Just to mention that a WebKit version of this patch is already attached in this bugzilla, waiting for a final review:
https://bugs.webkit.org/show_bug.cgi?id=129313
It's just pending, IIRC, in answering some final doubt exposed in there on how to write the new test for that specific case. My vote goes for useing regular tests to complete the backport and think about converting all of them to reftests later.
Ryosuke, would you mind commenting on that?
https://bugs.webkit.org/show_bug.cgi?id=129313#c20
Thanks!